WASHINGTON, Dec. 21 (UPI) -- At 6:04 p.m. EST on Sunday, the sun will be appear directly overhead along the Tropic of Capricorn, at 23.5 degrees latitude, south of the Equator.
It's the winter solstice, marking the beginning of winter and the longest day of the year in the Southern Hemisphere and the shortest day of the year in the Northern Hemisphere. Naturally, being the shortest day of the year, Sunday night will be the longest night of the year for the top half of planet Earth.
More than that, Sunday night will be longest in over a century. Scientists estimate that the Earth's rotation has slowed nearly every year since our planet first formed 4.5 billion years ago. While many thought that would mean tonight would be the longest night in history, it is actually only the longest since 1912, as the Earth has sped back up slightly in the decades since.
The slowdown is caused by a phenomenon known as tidal accelerations, whereby Earth's tidal bulge is pushed ahead by the planet's rotation. The bulge acts as a boost to the Moon, pushing its orbit slightly farther away, while the friction of the offset tidal bulge slows the rotation of the Earth. The effects are infinitesimal, but extremely precise atomic clocks have allowed scientists to confirm the slowdown.
That's the complicated part. As to why every December 21 or 22 is the shortest day and longest night of the year in the north and vise versa in the south -- that's easy. Because the Earth rotates along a tilted axis, the amount of sunlight different places on Earth receive over the course of a day changes as the Earth orbits around the sun. The roles of the two hemispheres are reversed on June 20 or 21 during the summer solstice, when Northern Hemisphere residents enjoy their longest day of the year.
While the two solstices mark the longest and shortest days of the year, they rarely ever mark the warmest or coldest days or nights. For those north of the equator, the days will be continue to longer and longer for the next six months, but because the ocean is slower to heat up and cool down, it will be several more weeks before the ground and ocean reach equilibrium and the top half of the planet can begin to take advantage of the extra solar energy.